Curious what the purpose of this duct is and whether I can close it off. It goes directly outside.

I was planning to convert this room to an office. The furnace and hot water heater are a few feet away from this opening. Today is our first cold day (26 degrees and snowing in MT) and I can feel the cold air beneath the opening.

Good Article, it all adds up and if you've sealed things up, you need to think about fresh air intake. As a huh? Wow! Consider this. My living room has three decorative wood beams spanning 14 ft. On each side, there was a 1/4" slot between the gypsum board that was only covered by the insulation vapor barrier. Not much til you run the numbers. 14' = 168", 168" * 0.25" = 42 sq.in. which multiplied by 6 gives you an opening of 252 sq.in. Ok, square root of 252 gives us a hole in the ceiling 15.87" on a side. That's a lot of hot air to lose. Caulk is the wallet's best friend.
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Fiasco LabsOct 29 '13 at 22:49